1. Field of the Invention
The invention is related to an improved skull processing method and apparatus for making crystals of high melting point substances such as zirconium dioxide or the like.
2. Description of the Prior Art
The skull melting process was developed to provide a means for heating refractory oxides to higher temperatures of up to 3000.degree. C. in air.
The skull process involves direct high frequency induction heating of a refractory material, for example. The material is contained in a hollow, water cooled crucible-like container. The container is generally made of copper, water is pumped through the container to cool the container and thereby cool the interface between the container and the material being heated therein.
A stable melt is formed in the center of the material and retained in a sintered shell (or skull) of an identical composition formed at the interface between the material and the container when the material is heated. The skull eliminates the problems of a reaction with the container or contamination of melt by the container. Reaction with the container is limited by the interface of the skull between the melt and the inside wall of the container. The melt is solidified by slowly decreasing the temperature of the melt to room temperature. The material comprising the melt nucleates and grows into single crystals during this cooling process. Skull melting as described is a known process.
In a modification of the skull process, a powdered refractory oxide charge, for example, of zirconium dioxide which melts at a temperature of 2690.degree. C. is placed in a finger-like metal water-cooled container. Melting is initiated by inductively heating the oxide particles. As the temperature of the oxide is increased, the oxide charge becomes electrically conductive and the radio frequency magnetic field (operating in the range 2.5 to 5 megahertz) couples directly to the contained melt. Even though the metal container is separated into fingers with spaces in between them, RF coupling to the melt is not very efficient for heating the melt because of the energy pumped away by water cooling. A significant amount of heat is absorbed by water cooling, which increases power consumption.
Various theoretical analyses of the skull melting process have indicated that for industrial applications, melt size and cost of power consumption are important considerations. Precise control of the melt temperature is unnecessary.
A prior art structure developed by Varian Inc., and described in their Bulletin SMS-A1) entitled "Varian Intermat Skull Melting System (Model A-1)" consists of a crucible-like structure of concentric water-cooled copper tubes. Sufficient cooling water is pumped from the copper base through each individual tube to maintain a cool solid sintered skull. The crucible-like assembly comprises two halves with each half being electrically isolated from the other to maximize coupling efficiency to the melt.
Water-cooled equipment has serious drawbacks that particularly affect the associated method of operation. Sometimes, the crucible-like container will leak water. If a crack develops on the interior portion of the crucible, the resulting water leak through the crack to the skull can result in an explosion. Tremendous amounts of power are necessary to maintain the melt temperature and at the same time enough water must be pumped through the crucible to remove sufficient heat from the skull to keep it solid. This makes water cooling inherently an extremely expensive step in the process because of the high cost of energy.
Frequently a skin forms over the top of the melt. This must be punctured manually to vent pressures from heated gases above the melt. This is a serious drawback and a potentially dangerous step.